THEORY
OF
OPERATION
CIRCUIT
DESCRIPTIONS
3-48.
RESISTANCE/CONDUCTANCE MEASURE-
MENT
PROTECTION
349.
Selecting
either the resistance or conductance
measurement
function
places Ql,
Q2,
and RT1 in parallel
with
the meter across
the resistance input
and
common
lines.
These
three components provide
protection
from
inputs
up
to
300V.
3-50. The
LO
RANGE O function of
the
8012A has a
separate
input
protection circuit rated to 300V. Refer to
the
schematic of the Low Ohms PCB
in
Section
8. The
protection
circuit must
protect two separate parts, the
10
mA
current
source and the input to the op amp circuit.
The op
amp input is
protected
up
to
300V
by R59, CR51,
and CR52.
The
10
mA current source is
protected
by the
circuit
formed by R51, R52,
Q51,
and
Q52.
This circuit
can
withstand up to 700V. Diode CR50
protects
the
current
source from
negative voltages.
3-51
.
CURRENT MEASUREMENT
PROTECTION
3-52. The mA input is
protected
two ways. Diode
CRl
and IC U4
limit the voltage drop
across
the
current
shunts.
Fuses FI and F2
protect
the current shunts
from
overcurrent
conditions. Fuse FI is
a
fast-blowing
fuse
rated
for
2A at
250V.
It is the
type with
a wire thread
in a
glass
envelope.
Under moderately
abusive overloads
FI
will
open to protect the circuits.
If
the overload voltage
is
high
enough, the
voltage
can
arc
through the metal vapor
caused by FI
blowing.
In
this
case, F2 opens. Fuse F2
is
rated for 3A
at
600V.
3-53.
The lOA
function of
the 8010A has no
fuse
protection circuitry.
However,
it can handle overloads
of
up to 12A.
3-54. Power Supply
WARNING
LINE
VOLTAGE POTENTIAL IS ON THE
PRIMARY POWER
CIRCUIT
ANY TIME THE
INSTRUMENT
IS PLUGGED INTO A
LINE
POWER
OUTLET.
3-55.
The
standard instrument can
be ordered with
a
power supply
that operates from 115V,
60
Hz;
lOOV,
50
Hz
or
60
Hz selectable; and 230V,
50
Hz, This
power
supply
provides
all basic operating
voltages
for the
instrument.
These voltages are rectified and
filtered
but
are not
regulated.
All regulated voltage
needs
are supplied
either
by U7
or by the
custom
IC,
U3.
The
POWER
switch
opens/closes
the secondary
circuit.
The power
supply
transformer (Tl) is a self-limiting transformer
and
therefore no input
line
fuse
is required.
3-56.
An
optional
rechargeable
battery pack
is available
for
both
Multimeters.
This option allows the Multimeters
to
operate from
90V to 264V,
50
Hz or
60
Hz (selectable).
Refer to
Section 6 for additional information on the
Rechargeable
Battery Option.
3-57. Touch
and
Hold
Circuit
3-58.
The touch and
hold
circuit
allows
the use
of
the
optional Touch and Hold
Probe
with your
instrument.
Refer
to
the Main
PCB
Assembly schematic in
Section 8.
The CL2
line to
the crystal Y1 continues to the left,
passing through
Q4
and
Q3
to
ground.
This
path allows
the crystal to oscillate. If
a
ground
is
applied
to the mA
terminal,
Q3
is
turned off, stopping the
instrument
clock.
The last
value displayed on the LCD remains
displayed
for
a few
minutes
if
the
mA
terminal
is
kept grounded.
CAUTION
The
Touch
and
Hold
function is meant to be
used for intervals of no longer
than
a
few
minutes. If the display is stopped for longer
periods of time (about 1 /2
hour)
the
outline
of the
digits
may remain
for a few hours
after the display is started again. Even
longer periods of time
may
cause
permanent
damage.
3-7/3-8
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